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Speaker impedance???

Discussion in 'Home Cinema Speakers' started by Wildviking, Oct 19, 2004.

  1. Wildviking

    Wildviking
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    Newbie question...

    My speakers specify an impedance, which does not match the impedance specified on the back of my amplifier. I have not perceived this as a problem, but am curious as to what this actually means. A low impedance (In very general terms=low resistance) means the speaker is 'tougher' for the amplifier to power, and vice versa? Thus, a lower impedance on the back of my amp than the impedance of the speakers means that the amp can supply more power than strictly required for the speaker...? And if so, a higher impedance on the amp means it probably may not match the speakers well? And finally, is it very important to match the impedance specified on the amp to that of the speakers, or is it only important to have a higher impedance on the speakers than on the amp? :lease:
     
  2. alexs2

    alexs2
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    This question appears very often here,and whilst a search in the forums should give you lots of answers,I hope this may help.

    As you rightly say,a low impedance is analogous to a low resistance,and thus places increasing demands on the amplifier's ability to deliver current into a decreasing load.
    Most amplifiers have their rated output quoted into 8 ohms,and relatively few will double their output into 4 ohms,and very few indeed will continue to do so as the load drops below 2 ohms.

    I wouldn't get too drawn into the issues of an amplifier being too powerful for a set of speakers...what's much more important is the amplifier's ability to deliver unclipped(undistorted with respect to maximum output)power into a given set of speakers.
    More speakers are killed off with low powered amps being overdriven,and producing vast amounts of high frequency distortion than those damaged by a very high powered amp being stuck on the front of a set of low power handling speakers.
    As an example,my B&W's are rated at 120W max,but are powered by amps capable of delivering vastly more.

    In broad terms,it's far safer to have a set of speakers with a relatively high impedance,and certainly within the amps rated range,than well below that,for the reasons given above.
    Most modern receiver/amps will drive 6ohm speakers quite happily,although some will struggle at 4ohms or less.....if you haven't had any problems,with the amp and speakers at your normal levels,then all should continue to be fine.
     
  3. Sniper

    Sniper
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    Amps don't have Impedance (not in the sense you mean!) - they do have an internal resistance but that's a totally different story!

    The impedance on your amp is just an indication to what speakers it will drive without ever clipping - clipping will occur if your powersupply can't keep up with the current being drawn & low impedance (4 ohm) speakers will draw more than higher impedance ones (8 ohm).

    If your amp is a 'cheapo' 8ohm one it will not like your 4ohm speakers at anything above 50% volume (and that might be pushing it). On the otherhand I've seen properly built '8ohm' amps powering '4 ohm' speakers without a sweat!
     
  4. alexs2

    alexs2
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    The internal resistance is output impedance,and will have a marked effect on frequency response,if it is very high,as in the case of e.g. SET amps,but as you rightly say,is another issue altogether!
    The other point you make about power supply is also spot on...a lot of budget amps have relatively poor power supplies,as this is a prime area of expenditure in the properly built ones.....essentially,asssuming the power supply voltage remains constant,as the load halves,the current draw doubles,and doing that is expensive(either in terms of switchmode supplies like Linn and Chord use,or conventional designs...Krell,Levinson etc)....but the ability to drive difficult or very low loads is still restricted to a fairly small number of amps.

    The older ones amongst us may remember the Apogee Scintilla(a ribbon design speaker)that had an impedance of about 1ohm overall,and at it's introduction,only the Krell KSA100 and Levinsons could safely run it without catching fire :D
     
  5. Sniper

    Sniper
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    did those amps use a nuclear power supply !?!? :laugh:
     
  6. alexs2

    alexs2
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    LOL...no,just a 2kW rated transformer and around about 200 000uF of capacitance(per channel)...it adds up to a pair of Coke can sized caps per channel,and about 70kgs of amp and transformer.
    I had one of these before settling on a set of KMAs that easily outperform the older KSA,but the weight,heat etc all add up.
    A bit off-topic,but the following link may amuse you,especially the bit near the bottom concerning power and current delivery from one of the S series amps,which didnt really have the capacity of the KMA's at low impedances.
    http://hem.passagen.se/xayide/krell/KrellAmplifiers.html
     
  7. Londondecca

    Londondecca
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    I have a vague memory that Apogee had some electronics designed to push up the impedance to 1+ Ohms
     
  8. alexs2

    alexs2
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    There were 2 models.....the original had an overall impedance of 1 ohm,and there was a 4 ohm model available as well,but later models were available as 1 ohm impedance only.

    Beautiful sound,not exactly WAF material,and very difficult to find suitable amps.

    Loads of info here...(the comment about it killing a Krell 250a isn't surprising,as this AV model was not designed with 1 ohm loads in mind,whereas the older Class A designs were).

    There's also a (rather short) list of amps that actually worked with the things....I first heard them with Krells in the mid 80s' and was duly impressed :D

    http://www.apogeespeakers.info/
     
  9. Sniper

    Sniper
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    Sorry Wildviking for hi-jacking the thread - have to ask a question myself! But do ask further if you're still confused!

    SO: I've recently noticed a couple of amps (Yamaha 2400 is one of them) which has an interesing (read perplexing) option in its setup menus. Basically you can tell the amp if you're running "8ohm" speakers or "<=6ohm" speakers! What would this be actually (electically/electronically) acheiving? I mean a PSU can or cannot supply the required current (there are no two ways around that!). Is it merely telling the PSU "Yo Get ready for this..." or what? I wonder if it just 'caps' your max volume to a non-clipping level (i.e. the PSU is only up to 70% of the volume so it caps it at that level!)

    Anyone??

    UPDATE: I've had a look at the NAD site. They're always saying that they can drive anything down to 2 Ohms. One of the articles on how they design the PSU says "Many less advanced receivers resort to performance robbing protection resistors to allow the use of 4 ohm speakers."

    HMN maybe that's what these amps are doing! Placing resistors in series with your speakers to protect current draw!! NASTY!
     
  10. Wildviking

    Wildviking
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    Thanks for the info Sniper and everyone else who responded. I think things are a bit clearer now, basically what you're saying is that an amp rated @ 8 Ohms can comfortably run speakers rated at 4/6 Ohms, as long as the PSU in the amp is decent, and I'm a bit careful when turning up the volume...?
     
  11. alexs2

    alexs2
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    Also apologies for going a bit OT there....most amps will be quite OK with 6 ohm speakers,and should have no problems,especially more modern ones,which usually shut down at the first signs of trouble.

    Problems usually result from running amps with marginal delivery into 4 ohms at high levels...a good guide is what your amp's power delivery is into 8 and 4ohms.....if the figure rises,or ideally doubles at 4 ohms,then you should be fine...this is often quoted on the manufacturer's spec sheet,or alternatively,do a search for an online review.
     
  12. lynx

    lynx
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    Many amps, when switched to drive low ohmic value speakers, e.g. 4&#937; simply offer protection to the power transformer and thus limit the available current at the output devices.They are in effect 'throttling back' to save stressing the amp.
     

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